The field of digital television transmission, especially that transmission employing bandwidth compression techniques is summarized in "Digital Television Transmission Using Bandwidth Compression Techniques" by Kneko et al. in the IEEE Communications Magazine for July, 1980 at pages 14-22; and in "Digital Encoding of Color Video Signals--A Review" by Limb et al. appearing in the IEEE Transactions on Communications, Vol. COM-25, No. 11, November 1977 at pages 1349-1385. Unencoded digitized NTSC signals produce an approximately 86 Mbps bit stream. It has long been recognized that bandwidth compression techniques (for example DPCM) can be used to reduce that relatively high rate. For example, Ishiguro, in U.S. Pat. No. 3,843,940 discloses a "Differential Pulse Code Modulation Transmission System" for NTSC, SECAM or PAL video signals. Other examples of bandwidth compression techniques for color television are found in "Interframe Coding for 4 MHz Color Television Signals" by Iinuma et al. appearing in the IEEE Transactions on Communications, Vol. COM-23, No. 12, December 1975 at pages 1461 et seq; "1.544 Megabits per Second Transmission of TV Signals by Interframe Coding System" by Yasuda et al. appearing in the IEEE Transactions on Communications, Vol. COM-24, October 1976 at pages 1175 et seq; "A 32 Megabit per Second Component Separation DPCM Coding System for NTSC Color TV" by Swada et al. appearing in the IEEE Transactions on Communications, Vol. COM-26, No. 4 for April 1978 at pages 458 et seq and "32 Megabit per Second Transmission of NTSC Color TV Signals by Composite DPCM Coding" by Sawada et al. appearing in the IEEE Transactions on Communications, Vol. COM-26, No. 10 for October 1978, pages 1432 et seq.
The extent to which bandwidth compression is applied and hence the ultimate data rate of the coded video depends on constraints such as desired picture quality, cost and complexity of equipment and expected mode of transmission. While captive systems have been proposed at bit rates in the range of 1.5-32 megabits per second, it is generally expected that cable and/or network quality television signals will require higher data rates especially if intraframe, and not interframe, coding is used.
For applications of digital transmission techniques to existing video systems, one is constrained by the fact that the video signal is necessarily asynchronous to the transmission clock. Even in the general case, however, since the digital transmission is but an adjunct to the generation and use of the video signals, it should not be expected that the transmission system clock rate can be imposed on the video signal generation and thus, even in general, one should expect that the video signal will be asynchronous to the transmission rate clock.
There are, at the present time, substantial quantities of digital transmission equipment which adhere to common carrier standards. There is for example a T1 standard at 1.544 megabits per second, a T2 standard at 6.13 megabits per second and a T3 standard at 44.736 megabits per second.
Prior art suggestions for transmitting digitized video tie the transmission rate and format to the video line rate, see in this regard the Sawada et al. publication cited above, particularly Vol. COM-26, No. 4, page 462, wherein for transmission purposes, two different frames are defined, a first frame type is exactly equal to two adjacent horizontal line signals and a second frame type is exactly equal to a single line signal. See also Vol. COM-26, No. 10, page 1435.
This technique, while having an appearance of simplifying equipment, actually imposes unnecessary burdens on the transmission equipment. If, for example, standard data rate and format were used to encode the digital television, existing transmission monitoring equipment could be used to monitor the error rate of the data using parity bits provided for in the standard frame format. This practical consideration saves the cost of additional overhead in the transmitted data to perform the monitoring function, thus reducing the overall data rate required.
It is therefore an object of the present invention to provide a method and apparatus for transmitting digitized NTSC signals via standard transmission equipment using standard frame format and rate.
Since the video is DPCM encoded, the receiver requires a decoder to recreate the original video. This decoder can be simplified if it can recognize each video line start. Since the video timing is asynchronous to the T3 clock, we cannot use the transmission clock to identify video lines, as could be the case with the Swada proposals referenced above. Accordingly, we insert into the video stream digital codes uniquely identifying each video line start. Accordingly, it is another object of the invention to provide a method and apparatus for transmitting digitized video which has the capability of inserting code words uniquely identifying each video line start.
To further assist in properly receiving the encoded digitized video, the transmission includes information relating the color subcarrier (or video sampling clock) to the T3 clock. Since the T3 clock is available at both transmitter and receiver, this data can be used to meter the color subcarrier at the receiver to ensure it tracks the color subcarrier at the transmitter.